There are many exchanges other than Bitfinex and many different places to buy and sell.  Also, while Bitfinex does have a fairly high volume, it's not really comparable to Mt Gox, which had the vast majority of all trading volume.

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Still, your general point that Bitfinex is likely to be insolvent is reasonable.  Bitfinex has been promising an audit since partway through 2016 and has repeatedly failed to deliver, even trying to pass off Friedman LLP's recent information as an audit despite the fact that they explicitly stated it was not an audit and that they could not verify whether USDT have as access to as much money as they claim.

If you want more information about this, try reading through Bitfinexed's posts on Medium and Twitter.